Valves, such as butterfly valves and ball valves, for example, have a closing member, such as a disc or a ball, located within the valve bore defined by the valve body. The closing member is supported on one or more valve stems for rotational motion between an open position, allowing flow through the valve bore, and a closed position, preventing flow. At least one valve stem is used to effect rotation of the closing member, and therefore, the valve stem must penetrate the valve body to enable it to be turned to externally effect closing member rotation. The stem may be turned manually or by an actuator for example. Typically The valve stem is accommodated in a passageway extending through the valve body from the valve bore.
The passageway is subjected to the fluid pressure within the valve bore and must be sealed to prevent leakage through the clearance in the passageway between the valve stem and the valve body. Traditionally, the valve stem seal is effected by a valve packing, for example, a fibrous material, a compressible elastomeric material or a polymeric material arranged in the passageway between the valve stem and valve body. A follower engages the valve packing and compresses it to effect a fluid tight seal within the passageway. The follower is forced into the packing by means of integral threading of a flange bolted to the valve body, the packing compression being effected by applying torque to the follower or the flange bolts. As the seal wears, more torque is applied increasing the compression of the packing to stop any leaks.
Seals formed by traditional packing are not effective if the valve body within the passageway becomes corroded. The packing cannot form a long lasting fluid-tight seal against a corroding surface regardless of how tightly it is compressed within the passageway. The corrosion continues to eat away at the sealing surface rendering the packing ineffective. The leak even accelerates the corrosion process, and if it is desired to properly seal the valve, the valve must be disassembled, the corrosion removed and the packing replaced.
Yet another problem associated with valves having valve stems is the lack of adjustability with regard to valve closing member positioning within the valve bore. This is especially problematic for the butterfly valve which has a closing member comprising a slanted disc, wherein the face of the disc is angularly oriented with respect to the valve bore. The slanted disc has a circumferential elastomeric seal which interfaces with a circumferential seat on the valve bore.
The disc is supported on two valve stems arranged diametrically oppositely from one another across the disc. Valve stem bearings are associated with the valve stems to permit relative rotation between the stem and the valve body or the disc and the valve stem. Due to the accumulation of tolerances between the valve body, valve stems and valve disc, the disc is usually not perfectly centered within the valve bore upon assembly. Centering of the disc is necessary to ensure proper functioning of the valve seal, to provide even wear of the seal to maximize valve life, to minimize the operating torque required to actuate the valve and to prevent undesired contact between the disc and the valve bore at points other than between the valve seal and seat.
Centering of the disc within the valve bore is typically accomplished by means of shims. This is a time consuming activity which must be done by hand and, therefore, increases the cost of valve assembly. Furthermore, the accuracy of the shimming is limited by the accuracy of the shims used and the process by which the required shims are determined. The inherent lack of shimming accuracy may lead to imperfectly centered discs and/or allow for unwanted free motion of the disc in a direction along the axis of the valve stems. These problems lead to accelerated valve failure as explained below.
When the slanted disc is in the closed position or partially opened in a "throttling" condition, the differential fluid pressure within the valve bore causes a load on the disc which is purely perpendicular to the face of the disc. Since the disc is angularly oriented with respect to the bore and the valve stems, the load on the disc is conveniently resolved into two components, one component being perpendicular and the other being parallel to the valve stems. The load component perpendicular to the valve stems is reacted by the aforementioned valve stem bearings and the valve stems. However, any free motion in the disc along the axis of the valve stems due to tolerance stack up and shimming inaccuracy will allow the disc to shift within the valve bore in response to the load component parallel to the valve stems. The load parallel to the valve stems will compress one segment of the disc against the valve seat, increasing the compressive force between the circumferential seal and the seat along that segment. The same load simultaneously pulls an opposing segment of the disc away from the seat, reducing the compressive force between the circumferential seal and the seat along that opposing segment.
The segment of the seal seeing the greater compressive force tends to wear faster upon valve actuation than the segment of the seal diametrically opposite on the disc. This leads to accelerated seal failure and shorter valve life. It may also result in metal to metal contact between the valve disc and the seat, causing damage to the seat and resulting in greater torque being required to open and close the valve. Furthermore, the portion of the seal seeing the reduced compressive force may not seal effectively against the seat and may tend to leak under the differential pressure in the valve bore.